Particle detection and analysis is desirable in a variety of manufacturing and environmental contexts. For example, in clean rooms used for the fabrication of integrated circuits, highly accurate particle detection is required due to the small dimensions of the devices under production. Significant failure rates in integrated circuits are associated with the presence of particles greater than one-tenth the device linewidth. Typically, the smaller the size of the particle, the greater the number of particles which are present. Therefore, as linewidths decrease within the sub-micron range, particle removal becomes increasingly difficult and costly. Consequently, control of a particle source is usually more cost-effective than removal of particles once they are liberated from their source. Through real-time particle analysis, particle sources can be identified and controlled.
Particle detection and analysis in clean rooms and gas distribution systems is typically done by real-time, also known as on-line, counting of airborne particles followed by off-line analysis of deposited particles by microscopic or laser scan techniques. The former technique provides the rapid response required for monitoring particle generation events while the latter technique provides size and elemental composition information. Particle counting is frequently performed using standard light-scattering particle counters. However, these devices can only detect particles and provide no compositional information. While off-line analysis provides particle composition information, it is also limited by the particle sizes it can detect and cannot be time-correlated to particle generation events.
Mass spectrometry is an analytical technique used for the accurate determination of molecular weights, identification of chemical structures, determination of mixture compositions, and quantitative elemental analysis. Molecular structure is typically determined from the fragmentation pattern of ions formed when the molecule is ionized. Element content of molecules is determined from mass values obtained using mass spectrometers. However, since mass spectrometers typically operate in vacuum, particulate analysis usually requires that nearly all of the particular carrier be separated from the particulate material prior to ionization in the spectrometer. This requirement increases the complexity of particle detection for particulates suspended in liquids and gases
Laser-induced mass spectrometry is described in U.S. Pat. No. 5,382,794 issued Jan. 17, 1995, commonly assigned to the instant assignee, the disclosure of which is incorporated by reference herein. In the patent, an exemplary laser-induced mass spectrometry system is described in which particles enter an evacuable chamber through an inlet device such as a capillary. A laser, such as a pulsed laser is positioned such that the laser beam intersects the particle stream. As the particles pass through the path of the laser beam, they are fragmented and ionized. A detector, such as a time-of-flight mass spectrometer detects the ionized species. Mass spectra are produced, typically being recorded with an oscilloscope, and analyzed with a microprocessor. The mass spectra information permits real-time analysis of the particle size and composition.
While the laser-assisted spectrometry system described in the patent provides useful real-time particulate analysis, there is a continuing need to provide compositional and size evaluation for increasingly smaller particulates. There is a further need in the art for detection and analysis of a greater percentage of the particulate contents of a sample, to ensure accurate characterization. Finally, there is a need in the art for particulate analysis systems and techniques which do not discriminate against high electronegativity and high ionization potential elements. U.S. Pat. No. 5,361,462, issued May 20, 1997, commonly assigned to the instant assignee, the disclosure of which is incorporated by reference, solves these problems by introducing the particles into the chamber through a capillary tube. A laser is positioned to produce a focused laser beam which intersects the particle laden gas stream at a position approximately 0.05 mm to 1.0 mm from the chamber entrance. The laser beam has a power density sufficient to fragment and ionize particles entrained within the particle-laden gas stream. Although this method and apparatus does not discriminate against high electronegativity and high ionization potential elements, further improvement in the probability of particles being ionized for detection is desired.